The present invention relates to a digital noise reducer suitable to use in a video tape recorder or the like.
As apparatuses for reducing noises of picture signals, so-called noise reducers using correlation (1) between lines, (2) between fields, (3) between frames, or the like in picture signals are often used. Owing to the advancement of semiconductor techniques in recent years as well as development of high-speed A-D (analog-digital) converters, D-A (digital-analog) converters and memories applicable to picture signal processing as well, realization of noise reducers using digital techniques has also become possible. In particular, field delays and frame delays have become possible by using memories as delay circuits. Noise reducers using the field correlation or frame correlation which have heretofore been impossible with analog circuits can thus be put into practical use.
These noise reducers using digital techniques are discussed in "Gazo no digital shingo shori (Digital signal processing of pictures)" written by Fukinuke and published by Nikkan Kogyo Shinbunsha, pp. 115 to 118. An example thereof will now be described by referring to FIG. 1. In FIG. 1, numeral 1 denotes an addition circuit, 2 a subtraction circuit, 3 a coefficient multiplication circuit, 4 an input terminal, 5 a delay circuit, and 6 an output terminal. Processing is performed by means of digital means, and the delay circuit 5 comprises a memory and its control circuit.
Assuming now that a signal (digital datum) input to the input terminal 4 at time t.sub.1 is Xi, the input signal Xi is supplied to the addition circuit 1 and the subtraction circuit 2. Concurrently therewith, a signal Yi is supplied from the delay circuit 5 to the subtraction circuit 2. In the subtraction circuit 2, the input signal Xi is subtracted from the signal Yi supplied from the delay circuit 5. The resultant difference component (Yi-Xi) is supplied to the coefficient multiplication circuit 3. In the coefficient multiplication circuit 3, the difference component (Yi-Xi) thus supplied is multiplied by K (0.ltoreq.K&lt;1). The resultant K(Yi-Xi) is supplied to the addition circuit 1. In the addition circuit 1, the input signal Xi supplied from the input terminal 4 and the difference component K(Yi-Xi) supplied from the coefficient multiplication circuit 3 are added together to form a signal Zi. This signal is output from the output terminal 6 and supplied to the delay circuit 5. In the delay circuit 5, the supplied signal Zi is delayed by a predetermined time, (such as one field interval in an apparatus using field correlation, for example) and supplied to the subtraction circuit 2. The addition circuit 1, subtraction circuit 2, coefficient multiplication circuit 3 and delay circuit 5 constitute a so-called noise reducer of feedback type.
This noise reducer of feedback type has transfer function H(Z) represented by equation (1) as: ##EQU1## wherein Z.sup.-1 is a unit delay operator and is related to a delay time T of the delay circuit 5 as: EQU Z.sup.-1 =e.sup.-jwT. (2)
The noise reducer of feedback type having the transfer function H(Z) of the equation (1), where the unit delay operator Z.sup.-1 is represented by the equation (2), exhibits comb-shaped frequency response, which assumes peaks at frequencies n/T (where n=0, 1, 2, . . . ) and which assumes bottoms at frequencies (2n+1)/2T as shown in FIG. 2. Assuming that T is a field period or a frame period and the input signal input from the input terminal 4 of FIG. 1 is a video signal, its spectrum distribution coincides with the peaks of FIG. 2. Therefore, noise components in the video signal which coincide with the bottom portions are reduced.
That is to say, by making the delay time T of the delay circuit 5 equal to one line interval, one field interval or one frame interval, the frequency spectra of the picture signal coincide with peaks of FIG. because of the correlation property of the picture signal. Noise components having no correlation coincide with the bottom portions. Accordingly, the picture signal input from the input terminal 1 is obtained at the output terminal 6 without attenuation, whereas noises located at bottom portions are reduced. As a result, the S/N ratio is improved.
The depth of bottom portions of the comb-shaped frequency response shown in FIG. 2 depends upon the feedback coefficient K. When the feedback coefficient K is small, the bottom portions are shallow as indicated by broken lines. When the feedback coefficient K is large, the bottom portions become less shallow as indicated by solid lines. The larger the feedback coefficient K becomes, therefore, the larger the noise reducing effect becomes.
On the other hand, the signal Zi obtained at the output terminal 6 of FIG. 1 can also be represented as: ##EQU2## where the signal Yi is an output signal obtained before the input signal Xi by the delay time of the delay circuit 5. Assuming now that there is difference .DELTA.Xi between the input signal Xi and the output signal Yi, the input signal can be represented as Xi=Yi+.DELTA.Xi. By substituting this equation into the equation (3), we get: EQU Zi=(1-K).DELTA.Xi+Yi. (4)
As the feedback coefficient K is made large in a noise reducer of feedback type, the amount of noise reduction becomes larger as described above. When the feedback coefficient K (where 0.ltoreq.K&lt;1) is large, however, it is evident from the equation (4) that (1-K).DELTA.Xi becomes small, and the difference component .DELTA.Xi satisfying the relation: EQU .vertline.(1-K).DELTA.Xi.vertline.&lt;1 LSB (5)
(where 1 LSB is a digital value representing one gradation of quantization) is discarded in the process of rounding operation, and the output signal does not change in some cases. That is to say, an increase in feedback coefficient K causes increased degradation errors in a region where the video signal does not change often, resulting in deteriorated picture quality as if the number of quantization bits is substantially reduced.
In a line noise reducer using vertical correlation, for example, deterioration of vertical resolution (vertical blur) is caused at contour portions of the picture. In a frame noise reducer using frame correlation, deterioration (lag) of dynamic resolution depending upon a difference in picture contents between frames of a moving picture is caused. Further in a field noise reducer using field correlation, deterioration of vertical resolution and dynamic resolution is caused because the field noise reducer has intermediate characteristics between those of the line noise reducer and the frame noise reducer.